1. Field of the Invention
The present invention relates to a tire for motorcycles and, more particularly, a motorcycle tire which can enhance the running stability not only on conventional ordinary roads but also on such a relatively newly developed road which has rain grooves formed on the surface, without any sacrifice caused to fundamental characteristics of the motorcycle tire such as performances under high-speed running conditions, operation characteristics under wet road conditions, and so forth of the motorcycle.
2. Description of the Prior Art
In order to secure desirable performance characteristics such as above referred to, conventional motorcycle tires are normally structured such that in a portion of the tire which contacts the ground or road surface when a motorcycle mounting the tire is run straight forward, the circumferential grooves formed in the tire tread portion comprise, for example, a first main groove which is located in a central portion of the tread portion and which is straight and circumferentially continuous, and two second main grooves which are formed one run at a side of the first main groove and which have a zigzag pattern and circumferentially continuous.
On the other hand, there have of late been such roads developed in expressways or some of ordinary roads which have a number of grooves formed on the road surface (inter-groove pitch on the order of 19 mm) in a substantially parallel arrangement along the direction in which the road runs. These grooves are called rain grooves and are provided so as to improve drain between vehicle tires and their contacting road surface when the latter is in a wet condition due to rainfall for example. When a high speed running is made with conventional motorcycle tires having the above-mentioned designed grooves, on a road having the rain grooves, there tends to occur a problem such that on account of an interaction of the rain grooves on the road surface and the circumferential grooves on the tire, the vehicle mounting the tire undergoes lateral oscillation or vibration, whereby the vehicle driver is not only given unpleasantness but also subjected to a physical danger.